I'm guessing that the term "Michigan Lake Driftwood" is the best, most palatable, most marketable descriptive name they can come up with...and that it actually means a plain, ordinary piece of wood somebody picked up on the beach at the cottage. Not that there's necessarily anything wrong with that, I have grabbed wood like that for many years myself, but that kind of wood could have spent years (ideally) submerged before washing up on the beach ...or it might have been in the water for only a few weeks or months. If that's the case it still contains a great deal of sap and other substances that are, to put it plainly, decomposing. The fact that there is so much tannin being released points towards that conclusion.
The wood itself is mostly cellulose, which is an organic substance, but it can last many years underwater and degrades very slowly. The other stuff? Not so much. It's rotting and that's what you smell. Given enough time the smell will go away, but that doesn't happen quickly. You do plenty of water changes, so I am sure the fish won't suffer any great harm...but, really, is that fairly nondescript piece of wood worth putting up with the unpleasantness and concern?
If it were mine, I'd use it as part of a plant arrangement or some other non-aquarium project or decor, likely in a rock garden or around my backyard pond. I'd get something a little more friendly and a lot less stinky for my tank. If it were some spectacular, one-of-a-kind showpiece that I just had to use in the tank, I'd toss it in the natural pond on my land, weighted down to keep it on the bottom and with a stake marking its location...and I'd leave it there for a couple years to let nature take its course.
Yeah...I'm patient....
